Sheet piles are used in civil engineering to support vertical loads and/or to resist lateral pressures. The piles are sunk or driven into the ground. Sheet piles are formed from individual piles, generally formed of steel, which are interlocked or otherwise joined together to form a “wall”.
Uses for a sheet pile wall include use as a retaining wall, for example, to hold back water or earth. Walls constructed using sheet piling are used as both temporary and permanent structures and will generally be substantially water tight. A common use of a sheet pile retaining wall would be as a cofferdam, for example, in construction of a below water-level structure or a below ground structure. Another example would be as part of a bridge pier in a river.
Traditionally individual piles have each been formed in either a generally “U-shape” or a generally “Z-shape” with the web of the “Z” approximately 120° to 135° to the flanges. It will thus be appreciated that when the piles are joined together, with each alternate pile facing the opposite direction, they form a sheet pile having a cross section which approximates a square wave. The steel piles are generally formed by hot rolling, in a steel mill, but may also be cold formed. The depth of the pile is determined by the depth of grooves in the rolls, in the steel rolling mill. For the depth of the pile to become greater, then the depth of the grooves in the rolls must be increased. The traditional width of a pile was 600 mm to 700 mm.
As will be appreciated, the rigidity of the sheet pile is a critical parameter. The conventional indicator of rigidity is the section modulus of the steel pile or sheet pile. To increase the section modulus, it is desirable to increase the depth of the pile, but because of the limit to the depth of the grooves in the forming rolls of the steel mill it becomes more difficult to roll piles with a deep profile. In practice there is a maximum depth of approximately 500 mm that can be rolled, without the grooves in the rolls being so deep that they tend to weaken the rolls to the point where they can break.
Solutions to the requirement for a high modulus have been sought. Current solutions include using “Z-shape” or “U-shape” profiles in combination with “H-shape” profiles or tubular profiles to create a wall. As will be appreciated, these walls can take a variety of forms, depending on the arrangement of the piles and the requirements of the wall. To achieve the highest modulus, the tubular or “H-shape” profiles will generally be oriented such that the webs of the profiles are substantially at 90° to the plane of the front of the wall. This provides the highest level of stability and strength. However, the depth of a wall is an increasingly important factor in the construction of sheet piling walls and walls having tubular or “H-shape” profiles will inevitably add substantial depth to the wall due to the orientation of the piles. The depth of the wall is particularly important in situations where space is limited, for example when forming coffer dams for city basements. Here a greater depth of pile is a significant penalty to owners of buildings due to the consequence of a loss of useable space. Accordingly, a compromise will often have to be made between the depth of a wall and the strength.
An alternative solution to the requirement for a high rigidity is the strengthening of the sheet piles of a wall through the use of plates, which are welded to the sheet piles. These plates can be added to the traditional “Z-shape” or “U-shape” steel piles, or to the solutions involving “H-shape” or tubular profiles. However, the additional welding and materials that are required will increase costs and labour requirements. Furthermore, the weight of the pile will be substantially increased. Similar issues also arise when constructing “H-shape” or tubular piles. For completeness, it should be mentioned that another factor is the thickness of the steel, both at the flanges and webs and at the extreme edges. Obviously, greater stiffness can be achieved by using a thicker steel section, but this is undesirable since it leads to a heavier and more expensive section. In the above discussion, a steel section of standard thickness is being assumed (generally 20 mm).
A method for manufacturing sheet piling is taught in WO-A-99/42669. The method teaches that an “open Z-shape” piling or an “open U-shape” piling manufactured by hot forming steel sections can be cold reformed to decrease the angle between the flanges and the web so as to increase the depth of the section and increase its section modulus, while still maintaining at least some of the advantage of the width of an “open Z-shape” or an “open U-shape”.
Where a sheet pile wall is used as a retaining wall, for example against a flow of water, the wall may be strengthened or optimised using additional materials and/or components. For example, in a river, the wall requires a smooth face for hydraulic flow and a slender section to minimise impedance to the flow of water and resistance to abrasion and corrosion. This is typically achieved through casting concrete in situ. Where a high modulus retaining wall is required in marine applications, the sheet piles are often connected to large steel pipes having diameters of approximately 1 m to 2 m. These retaining walls are known as “Combi” walls. However, these walls cannot be concrete clad due to their shape. Thus, they often suffer from abrasion and corrosion, resulting in significant maintenance costs.